The present invention relates generally to fuel cell systems and, more specifically, to silicon electrode structures and silicon electrode assemblies associated with fuel cell systems.
A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or methanol, to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell system can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cells employing methanol as the fuel supplied to the anode (also commonly referred to as a xe2x80x9cDirect Methanol Fuel Cellxe2x80x9d (DMFC) system), the electrochemical reactions are essentially as follows: first, a methanol molecule""s carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule""s oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air supplied to the cathode is reduced to anions with the addition of electrons. From a molecular perspective, the electrochemical reactions occurring within a direct methanol fuel cell (DMFC) are as follows:                             ⁢                  Anode          ⁢                      :                                                                                  CH              3                        ⁢            OH                    +                                    H              2                        ⁢            O                          →                              6            ⁢                          H              +                                +                      6            ⁢                          e              -                                +                                              E          0                =                  0.04          ⁢                      xe2x80x83                    ⁢          V          ⁢                      xe2x80x83                    ⁢                      vs            .                          xe2x80x83                        ⁢            NHE                    ⁢                      xe2x80x83                    ⁢                      (            1            )                                                  xe2x80x83                            CO        2                            xe2x80x83                                        ⁢                  Cathode          ⁢                      :                                                      ⁢                                                            3                2                            ⁢                              O                2                                      +                          6              ⁢                              H                +                                      +                          6              ⁢                              e                -                                              →                      3            ⁢                          H              2                        ⁢            O                                                        E          0                =                  1.23          ⁢                      xe2x80x83                    ⁢          V          ⁢                      xe2x80x83                    ⁢                      vs            .                          xe2x80x83                        ⁢            NHE                    ⁢                      xe2x80x83                    ⁢                      (            2            )                                                          ⁢                  Net          ⁢                      :                                                      ⁢                                                            CH                3                            ⁢              OH                        +                                          3                2                            ⁢                              O                2                                              →                                                    H                2                            ⁢              O                        +                          CO              2                                                                    E          0                =                  1.24          ⁢                      xe2x80x83                    ⁢          V          ⁢                      xe2x80x83                    ⁢                      vs            .                          xe2x80x83                        ⁢            NHE                    ⁢                      xe2x80x83                    ⁢                      (            3            )                              
The various electrochemical reactions associated with other state-of-the-art fuel cell systems (e.g., hydrogen or carbonaceous fuel) are likewise well known to those of ordinary skill in the art.
With respect to state-of-the-art fuel cell systems generally, several different configurations and structures have been contemplatedxe2x80x94most of which are still undergoing further research and development. In this regard, existing fuel cell systems are typically classified based on one or more criteria, such as, for example, (1) the type of fuel and/or oxidant used by the system, (2) the type of electrolyte used in the electrode stack assembly, (3) the steady-state operating temperature of the electrode stack assembly, (4) whether the fuel is processed outside (external reforming) or inside (internal reforming) the electrode stack assembly, and (4) whether the reactants are fed to the cells by internal manifolds (direct feed) or external manifolds (indirect feed). In general, however, it is perhaps most customary to classify existing fuel cell systems by the type of electrolyte (i.e., ion conducting media) employed within the electrode stack assembly. Accordingly, most state-of-the-art fuel cell systems have been classified into one of the following known groups:
1. Alkaline fuel cells (e.g., electrolyte is KOH);
2. Acid fuel cells (e.g., electrolyte is phosphoric acid);
3. Molten carbonate fuel cells (e.g., electrolyte is 63% Li2CO3/37% K2CO3);
4. Solid oxide fuel cells (e.g., electrolyte is yttria-stabilized zirconia);
5. Proton or ion exchange membrane fuel cells (e.g., electrolyte is NAFION).
Although these state-of-the-art fuel cell systems are known to have many diverse structural and operational characteristics, such conventional systems nevertheless share common characteristics with respect to their electrode design. For example, conventional fuel cell electrode structures are generally constructed to serve two principal functions: (1) the first is to electrocatalyze the fuel or oxidizer, and (2) the second is to electrically conduct released electrons out of the fuel cell and to the electrical load. Because these two principal functions are generally not obtainable by a single state-of-the-art electrode material, most conventional electrode designs comprise a layered structure that includes, for example, a support substrate (e.g., a graphite or plastic plate having a flow field channel patterned thereon), a catalytic active layer (e.g., a carbon-fiber sheet or layer having affixed or embedded catalyst particles), and a current collector layer (e.g., a gold mesh) for the transmission of the generated electrical current. Such conventional electrode designs may be advantageous for vehicular and other larger scale power applications, but are problematic for smaller scale stationary applications such as, for example, miniature fuel cell systems for portable electronic applications. In short, conventional electrode platforms (with their several layers of disparate materials) are difficult to fabricate on a micro-scale basis.
Although significant progress has been made with respect to these and other fuel cell system problems, there is still a need in the art for improved fuel cell electrode structures and fuel cell electrode stack assemblies, as well as to methods relating thereto. The present invention fulfills these needs and provides for further related advantages.
In brief, the present invention relates generally to fuel cell systems and, more specifically, to silicon electrode structures and silicon electrode assemblies associated with fuel cell systems, as well as to methods relating thereto. In one embodiment, the present invention is directed to an electrode structure adapted for use with a fuel cell system such as, for example, a direct methanol fuel cell system. In this embodiment, the invention may be characterized in that the electrode structure comprises a silicon substrate having one or more selectively doped regions thereon, wherein each of the one or more selectively doped regions is adapted to function as a current collector for the transmission of an electrical current.
In another embodiment, the present invention is directed to an electrode structure adapted for use with a fuel cell system. In this embodiment, the electrode structure comprises a silicon substrate having one or more discrete porous bulk matrix regions disposed across a top surface, wherein each of the one or more discrete bulk matrix porous regions is defined by a plurality pores that extend into the silicon substrate, wherein the plurality of pores define inner pore surfaces, and wherein the inner pores surfaces have catalyst particles uniformly dispersed thereon.
These and other aspects of the present invention will become more evident upon reference to following detailed description and attached drawings. It is to be understood that various changes, alterations, and substitutions may be made to the teachings contained herein without departing from the spirit and scope of the present invention. It is to be further understood that the drawings are illustrative (hence, not necessarily to scale) and symbolic representations of exemplary embodiments of the present invention.